This invention relates generally to a closed system for pressure casting of lead articles and more specifically to pressure casting of lead and lead alloy parts while maintaining the supply of lead in a molten state without introducing air into the closed system even though a charge of molten lead is removed from one end of the system and a fresh charge of molten lead is introduced in the other end of the system.
Battery terminals are a typical type of part that is made of lead or a lead alloy and are usually cold formed in order to produce a battery terminal that is free of voids and cracks. If lead or lead alloy battery terminals are pressure cast, air is left in the battery terminal cavity in the mold so that as the lead solidifies, the air bubbles prevent the battery terminal from cracking. That is, the air bubbles act as fillers so the lead remains distributed in a relatively uniform manner throughout the battery terminal. Unfortunately, if air bubbles that form within the battery terminal are too large or numerous it can cause the battery terminal to be rejected. In order to minimize the formation air bubbles in a battery terminal, a vacuum can be drawn in the battery terminal cavity mold. The vacuum removes air from the mold and inhibits the forming of air bubbles in the battery terminal, but the battery terminals cast using a vacuum in the battery terminal cavity oftentimes solidify in an uneven manner producing battery terminals with cracks or tears which makes the battery terminals unacceptable for use.
In a process of pressure intensification, which is shown and described in my copending patent application Ser. No. 09/170,247 filed Oct. 13, 1998 and now abandoned, Titled APPARATUS FOR AND METHOD OF PRESSURE CASTING BATTERY TERMINALS and Ser. No. 09/208,795 Filed Dec. 10, 1998 and now issued as U.S. Pat. No. 6,202,733 titled APPARATUS AND METHOD OF FORMING BATTERY PARTS, a battery terminal is cast which is substantially free of cracks and tears by pressure casting a lead alloy while a vacuum is being applied to the battery terminal cavity. After the lead is directed into the battery terminal cavity, a piston is driven into the mold to rapidly reduce the volume of the mold for solidification. By precisely controlling the time of application of an external compression force to the molten lead in the battery terminal cavity, and consequently, the time at which the volume of the battery terminal cavity is reduced, one can force the molten lead or lead alloy in the flowable state into a smaller volume where the pressure on the battery terminal cavity is maintained. By maintaining the pressure on the battery terminal cavity during or after the solidification process, the battery terminal can be cast in a form that is free of cracks and tears.
In the present process, the aforementioned process of intensification can be coupled with a closed system that allows one to maintain the molten lead in the runners in a molten condition. Through the selective use of slidable pistons that seal against the walls of their respective cylinders one can remove a charge of molten lead from the closed system and also introduce a fresh charge of molten lead to the closed system without introducing air into the system. The system can be used with mold cavities that are evacuated, mold cavities that have an air bleed passage or mold cavities that have no air bleed passage. One of the slidable pistons is used to both increase the pressure of the molten lead in the system and draw molten lead into the system and the other piston forms part of a shut-off valve that opens and closes the flow of molten lead into the mold. If the end of the slidable piston that forms part of the shut-off valve is driven into the runner in the mold, one intensifies or increases the pressure of the molten lead in the mold. In these mold cavities where the air has not been evacuated, one can produce cast parts with air pockets of sufficiently small size so as not to have an adverse effect on the use of the part.
Briefly, the present invention comprises a system for molding lead articles wherein the system is maintained in a closed condition to prevent air from entering the molten lead in the system. The system includes a mold having a mold cavity with the mold maintainable at sufficiently low temperature so that a charge of molten lead located in the mold cavity solidifies to thereby form a solidified casting in the mold cavity. A housing having a runner for the flow of molten lead therethrough connects the mold cavity to a source of pressurizeable molten lead with the runner maintainable at a sufficiently high temperature so as to continuously maintain the molten lead in a molten state so that the mold cavity can be refilled with a fresh charge of molten lead from the runner when a solidified casting is removed from the mold cavity. The system includes a shut-off valve, having an open position for allowing molten lead to flow into the mold cavity and a closed position to prevent molten lead from flowing out of the runner as the molten lead in the cavity solidifies, and if needed an intensification mode to momentarily increase the pressure of the lead in the mold cavity to thereby minimize shrinkage and the size of voids or air pockets in the casting. When the system is coupled to an immersion housing a fresh charge of lead can be introduced into the closed system without introducing air into the supply of molten lead.